In the art of electronic imaging, particularly in the special application of film scanning, digitizing and storage, film images are scanned to produce a data output representative of the red, green, and blue (RGB) image signals. Such a scanner is disclosed in U.S. Pat. No. 5,045,932.
The scanning apparatus disclosed in U.S. Pat. No. 5,045,932 includes two types of linear array sensors. One is a full resolution imager spectrally sensitive to "luminance" like information, referred to as a full resolution luminance sensor. The other is a lower resolution tri-linear imager spectrally sensitive to red, green, and blue (RGB) image information, referred to as a half resolution tri-linear RGB sensor. A line being scanned is imaged onto the array through a beam splitter. Luminance information is generated by spectrally tailoring luminance channel sensitivity to approximate luminance. The luminance sensor "sees" light from red, green, and blue channels.
Scene luminance tonescale information is not provided by the luminance sensor. Signal processing is described which interpolates the color data up to full resolution, extracts detail information from the luminance channel, and adds the detail information to each of the R, G, B channels to form full resolution RGB scanned data. The RGB data is interpolated up to full resolution before processing, and the luminance sensor is used to generate detail information only. This full resolution RGB data can be further converted to luminance, chrominance (YCC) data through the use of look-up tables and color matrices. It is known to use a linear matrix to convert the full resolution RGB information to obtain luminance, chrominance information. Because the scanned luminance channel is formed in film transmission space, independent of the RGB sensor, the luminance channel provides image detail with incorrect scene tonescale information. In addition, the full resolution chrominance output must be subsampled for use in certain residual-based hierarchical storage and display devices.
Typically, the scanned data signals are compressed by known methods such as described in U.S. Pat. No. 4,969,204 to reduce storage requirements which provide for image decomposition and recomposition and make available reduced resolution versions of an original image for quick display. The method disclosed in U.S. Pat. No. 4,969,204 makes use of a full resolution luminance, Y, and two low resolution chrominance C.sub.1 C.sub.2 signals to encode an image in a more efficient color space than that using RGB signals.
This hierarchical image decomposition system accepts corrected, full resolution luminance and full resolution chrominance data as an input and provides a hierarchical family of reduced resolution image files as output. Chrominance data is subsampled by 2X before processing as full resolution chrominance is not stored or retained. The system recomposes an image for display by converting the YCC data back to RGB space through the use of look-up tables and matrices. Therefore the luminance data must carry tonescale information.
Scanning systems as in U.S. Pat. No. 5,045,932 and color space conversion systems as in the U.S. Pat. No. 4,969,204 can be used in combination to produce a YC.sub.1 C.sub.2 signal for hierarchical image compression. However, this process is overly complex because it scans with full resolution luminance and low resolution RGB sensors, converts the data to full resolution RGB, then converts the full resolution RGB to subsampled chrominance and full resolution luminance. In addition, such processed luminance information for encoding does not provide the same tonescale as luminance information generated from tonescale corrected RGB data.